Influence of B₄C Particle Size on the Microstructure and Mechanical Properties of B₄C/Al Composites Fabricated by Pressureless Infiltration

Abstract

To investigate the effect of B₄C particle size on the microstructure and mechanical properties of B₄C/Al composites, and to provide theoretical guidance for the subsequent thermal processing of composites, B₄C/Al composites with varying B₄C particle sizes (0.2 µm, 0.5 µm, 1 µm, 10 µm) were fabricated using pressureless infiltration. The microstructure of the composites was characterized using X-ray diffraction (XRD) and scanning electron microscopy (SEM), while the mechanical properties were analyzed by hardness test, three-point bending and high temperature compression. The results indicated that Al₃BC and AlB₂ were the primary interfacial reaction products in B₄C/Al composites, and interface reaction could be alleviated with increasing particle size. B₄C/Al composites with larger B₄C particle sizes exhibited a relatively uniform and discrete distribution of B₄C, while those with smaller B₄C particle sizes showed agglomeration of B₄C. The Vickers hardness and peak flow stress of B₄C/Al composites gradually decreased with the increase of B₄C particle size, while the bending strength, flexural modulus, and fracture toughness tended to increase. In addition, when B₄C particle size was 10 µm, the composites displayed optimal comprehensive performance with the lowest peak flow stress (150 MPa) and the highest fracture toughness (12.75 MPa·m^1/2).

1. Introduction

With the rapid development of the nuclear power industry, the neutron shielding properties of spent fuel storage and transportation materials need to be improved [1]. The high abundance of ¹⁰B renders B₄C a suitable material for ideal neutron radiation shielding [2]. In addition, B₄C has others attractive properties, such as ultra-high hardness, low density, and stable molecular structure, and is widely used in bulletproof armor [3], wear-resistant components [4], and refractories [5]. However, pure B4C presents challenges in densification through sintering [3,6] and exhibits poor fracture toughness (about 3.7 MPa·m^1/2) [7,8], which hinders its development and application. The above problems can be effectively solved by adding aluminum or aluminum alloy to form B₄C/Al composites, as aluminum exhibits excellent plasticity and possesses a density comparable to that of B₄C [9]. The neutron shielding performance of the B₄C/Al composites is approximately linearly proportional to the B₄C content [10], and increasing the proportion of B₄C can address the issue of insufficient storage capacity and excessive weight of spent fuel within the current protective apparatus.

Currently, B₄C/Al composites are mainly prepared by stirring casting [11], powder metallurgy [12] and infiltration techniques [13]. The infiltration method is the only approach capable of producing B₄C/Al composites with a B₄C content exceeding 55 vol.% [14]. Infiltration is divided into pressure infiltration and pressureless infiltration [15]. Pressure infiltration has limitations in terms of sample size and volume. Pressureless infiltration technology is a method that utilizes capillary force to facilitate the infiltration of liquid metal into porous ceramic preforms, thereby enabling the preparation of composites [16], which can prepare complex-shaped composites with high B₄C content [17].

The preparation of B₄C/Al composites by pressureless infiltration requires high temperatures to ensure wetting and spontaneous infiltration of liquid Al and B₄C, typically above 1000 °C [18]. However, high temperatures can lead to intense interface reaction and result in the formation of brittle phases, such as AlB₂, Al₃BC, AlB₁₂C₂, and Al₄C₃ [19]. The presence of numerous brittle phases can result in a decrease in fracture toughness and high temperature plastic deformation properties, subsequently impacting the hot working process [20].

Therefore, regulating interfacial reaction is crucial in the preparation of B₄C/Al composites with exceptional comprehensive properties. Li et al. [21] fabricated B₄C/6061Al composites by powder metallurgy at 560 °C and 620 °C, respectively; the interfacial reaction became more intricate and generated more products at 620 °C, which in turn diminished the aging hardening capability of the composites. Moreover, Liu Zhang et al. [22] utilized the spark plasma sintering (SPS) process to fabricate Ti-doped B₄C/Al composites, which effectively regulates the interfacial reaction of the composites, inhibits the formation of brittle phase, and significantly enhances their mechanical properties. Lai et al. [23] discovered that Sc, Zr, and Ti react with B₄C to form an interface layer, which hinders further reactions between B₄C and molten Al. Yingshui Yu et al. [24] utilized a simplified semi-continuous casting method and hot rolling process to fabricate B₄C/Al composites. The results indicated that an increase in the particle size of B₄C and a decrease in the mass fraction of fine B₄C led to a reduction in composite hardness, while ultimate tensile strength and impact strength were enhanced. In conclusion, the interfacial reaction of B₄C/Al composites can be regulated from various aspects, such as controlling the infiltration process (temperature and holding time) [25], adding alloy elements [26], and changing contact area between B₄C and Al [24].

To investigate the influence of contact area between B₄C and Al on microstructure and mechanical properties of B₄C/Al composites with high boron content, the composites were fabricated by means of pressureless infiltration with varying B₄C particle sizes. The effects of B₄C particle size on the microstructure, Vickers hardness, flexural strength, fracture toughness, and high temperature compression properties of the composites were examined.

2. Experiments

Commercial B₄C powder (99.9%, Beijing Zhongke Detong Technology Co., Ltd., Beijing, China) and aluminum particles (1 mm, 99.9%, Beijing Guantai Metal Materials Co., Ltd., Beijing, China) were utilized as the raw materials. B₄C with different particle sizes (0.2 µm, 0.5 µm, 1 µm and 10 µm) was weighed, mixed with 4 wt.% polyvinyl alcohol, and then put into the polyurethane grinding tank together with agate ball grinding beads. To achieve optimal dispersion, the mixed powder and an appropriate amount of reverse osmosis water were put into the QXQM-4 planetary ball mill (Suzhou Shanren Nano Technology Co., Ltd., Suzhou, China) with a speed of 180 rpm and mixed for 4 h. After ball milling, transfer the suspension from the tank to a metal plate, dry it in a drying oven at 80 °C for 10 h, then grind and screen, and pour the resulting mixed powder into a glass jar for storage.

The dried powder mixture was cold-pressed at a pressure of 373 MPa in a custom alloy mold, followed by loading the green billet into an argon-filled vacuum tube high temperature furnace (GSL-1750-KS, Hefei Kejing Material Technology Co., Ltd., Hefei, China) and preheating it to 1773 K for 2 h. The obtained preform is placed in the corundum crucible and wrapped with Al particles. Then the crucible is placed in the center of the tube furnace, followed by evacuation of the air from the furnace chamber and subsequent injection of a protective atmosphere consisting of Ar. Finally, the composites were obtained after infiltrating in argon atmosphere at 1473 K for 0.5 h before being cooled to room temperature in the furnace.

The phases of B₄C/Al composites were investigated by X-ray Diffraction (XRD, PANalytical, X’pert, Almelo, The Netherlands) with Cu-Kα radiation generated at 40 kV and 30 mA. The cross-section morphologies and phase distributions of the B₄C/Al composites were studied by Field Emission Scanning Electron Microscopy (SEM, JEOL, JSM-7100F, Tokyo, Japan) in backscattered mode.

In accordance with ASTM B962-2017, the actual density of B₄C/Al composites was determined using the Archimedes principle. The relative density was obtained by dividing the actual density by the theoretical density. Three examples with smooth surfaces were selected for testing each sample, and an average of three results was taken. Vickers hardness test was conducted on the automatic rotary digital Micro Vickers Hardness tester (HVS-1000Z, Shanghai Wanheng Precision Instrument Co., Ltd., Shanghai, China) in accordance with the national standard GB/T 16534-2009. Test parameters included a load of 1 kg and maintenance for 10 s. To ensure reliable results and minimize system errors, nine random points were selected on the surface of each sample for hardness testing, allowing for a comprehensive analysis of the composites. The measurement of bending strength for the composites was conducted according to the national standard GB/T 6569-2006. The test was performed using an electronic universal testing machine (E43.104, MTS Industrial Systems Co., Ltd., Shenzhen, China) in a three-point bending method, the size of the sample is 25 × 4 × 3 mm, and the lower span is 16 mm, with the indenter applying a pressure rate of 0.5 mm/min. The fracture morphology of the tested samples was obtained from the stress–strain curves and the fracture morphology of the specimens was analyzed. In addition, in accordance with the national standard GB/T 23806-2009, fracture toughness of the composites was tested by the electronic universal testing machine, and the size of the sample is 25 × 5 × 2.5 mm, the notch height is 2.5 mm, and the lower span is 20 mm. The fracture toughness of the composites was calculated to characterize the ability of resisting crack propagation. To provide the basis for the deformation of the material, such as via hot extrusion and rolling, the high temperature compression test was carried out with the Gleeble thermal simulation tester (Gleeble-3500, Dynamic Systems Inc., New York, USA) at a strain rate of 0.001 s⁻¹ and a temperature of 500 °C. The flow stress curve of the composites was obtained, and the plastic deformation behavior of the composites was characterized.

3. Results and Discussion

3.1. Microstructure

Figure 1 shows the XRD patterns of B₄C/Al composites with four different B₄C particle sizes (0.2 µm, 0.5 µm, 1 µm, 10 µm). The figure reveals the presence of four phases: Al, B₄C, Al₃BC, and AlB₂, where Al₃BC and AlB₂ are interfacial reaction products. The phase composition is basically consistent with previous research by Mi Chen et al. [27] and Tal Eller et al. [28], who also observed the generated interface products, such as AlB₂ and Al₃BC. Simultaneously, the diffraction peak intensities of Al and B₄C in the composites increase significantly as the particle size of B₄C increases from 0.2 µm and 0.5 µm to 1 µm and 10 µm. This suggests that the content of Al and B₄C in the composites also increases with the increase of B₄C particle size. On the contrary, the diffraction peak intensities of interface products Al₃BC and AlB₂ show a decreasing trend with the increase of B₄C particle size, indicating a decrease in the interfacial reaction products and a weakening of the interface reaction.

The observed results can be attributed to the larger surface area of B₄C powder with particle sizes of 0.2 µm and 0.5 µm, which enhance the contact area between B₄C powder and Al, leading to a more vigorous interface reaction at high temperature. Consequently, the composites with B₄C particle sizes of 0.2 µm and 0.5 µm are primarily composed of Al₃BC and AlB₂. Therefore, the intensity of the Al₃BC and AlB₂ diffraction peaks in composites with B₄C particle sizes of 0.2 µm and 0.5 µm are stronger. On the contrary, the interface reaction between B₄C and Al is alleviated due to the decreasing contact area between B₄C and Al as the B₄C particle size increased to 1 µm and 10 µm. In conclusion, the intensity of interface reaction between B₄C and Al decreases with the increase in B₄C particle size.

To understand the phase composition and distribution of the composites, Backscattered Electron (BSE) imaging was performed to characterize the samples. BSE images primarily reflect surface composition characteristics, with areas containing higher average atomic numbers appearing brighter on the fluorescent screen [29]. The main phases in the composites are Al, B₄C, Al₃BC and AlB₂, with average atomic numbers of 13, 5.2, 10, and 7.67, respectively [30].

Figure 2 shows the BSE images of four B₄C/Al composites with different B₄C particle sizes. Combined with the XRD results and the imaging principle of BSE, the regions of different phases are marked in the figures. The images demonstrate that B₄C is uniformly distributed throughout the composites, and no obvious cracks, holes and other defects are observed. The uniform distribution of B element and good interface bonding of the composites are conducive to improving the neutron absorption performance of the composites.

With the increase in B₄C particle size, the area of Al₃BC and AlB₂ decreases, while that of Al increases. This is attributed to the reduction in contact area between B₄C and Al, resulting in an alleviation of interface reaction, which is consistent with the XRD results. Besides, it is observed that the interface between B₄C and Al in B₄C/Al composites with particle sizes of 0.2 µm and 0.5 µm is obscure. In contrast, as the B₄C particle size increase to 1 µm and 10 µm, the interface becomes more distinct and the interspace between B₄C particles gradually expands, which suggests that interface reaction decreases along with the increase of B₄C particle size.

3.2. Mechanical Properties at Room Temperature

Table 1. Relative density test results of B₄C/Al composites with different B₄C particle sizes.

B4C Particle Size (μm)	Actual Density (g/cm3)	Theoretical Density (g/cm3)	Relative Density
0.2	2.555	2.601	0.982
0.5	2.555	2.602	0.982
1	2.577	2.600	0.991
10	2.552	2.599	0.982

shows the relative density test results of four B₄C/Al composites measured by Archimedean method. The calculation formula is as follows:

$$e=\frac{{\rho }_1}{{\rho }_0}$$

$${\rho }_1=\frac{m_1·{\rho }_{\mathrm{water}}}{m_2-m_3}$$

where 𝜌₁ and 𝜌₀ represent the actual and theoretical density of the composites, respectively; 𝜌_water is the density of water; and 𝑚₁, 𝑚₂, and 𝑚₃ denote the mass of the composites in air, after absorbing water from the air, and in distilled water. It can be observed that the relative density of the composites exceeds 98%, with no significant variation. The results suggest that the interface of the B₄C/Al composites exhibit excellent bonding.

The Vickers hardness values of B₄C/Al composites with varying B₄C particle sizes are presented in Figure 3. The average Vickers hardness values for B₄C/Al composites with particle sizes of 0.2 µm, 0.5 µm, 1 µm, and 10 µm are 627.18 HV, 585.67 HV, 236.22 HV, and 282.10 HV, respectively. This indicates that with the increasing of B₄C particle size, the Vickers hardness of the B₄C/Al composites shows a decreasing trend.

After sintering at high temperature, the new phases of the composites increased the grain boundary area, resulting in grain refinement strengthening, and the uniform distribution of B₄C particles in the matrix also produced the dispersion strengthening effect. The two strengthening mechanisms became decreasingly apparent as the size of B₄C particles increased [31]. In addition, combined with the XRD and BSE results, it is observed that, as the B₄C particle sizes were 0.2 µm and 0.5 µm, the Al₃BC and AlB₂ phases content increased in the B₄C/Al composites. The hardness values of the four main phases were ranked from large to small: B₄C, Al₃BC, AlB₂, and Al. That means that, with a higher Al₃BC and AlB₂ content, the Vickers hardness value of the B₄C/Al composites is higher. In summary, the gradual decrease of Vickers hardness values suggests a corresponding reduction in interface reaction with the increase in B₄C particle size.

Besides, the scattered blue diamond points in the figure represent the specific hardness value of the composites, and the standard deviation of hardness values for all samples is significant, with values ranging from 120.95 HV to 261.47 HV. These are consistent with the analysis of BSE, which indicates that, under high-temperature reaction conditions, B₄C/Al composites with different particle sizes tend to form regional reaction products instead of uniformly doping each phase together. Therefore, the test point may land on a single phase during Vickers hardness testing, resulting in a significant standard deviation. In addition, composites with B₄C particle sizes of 1 µm and 10 µm exhibited lower standard deviations due to milder interfacial reactions and a decrease in the content of Al₃BC and AlB₂.

The bending strength test is primarily utilized to assess a material’s ability to withstand bending forces, as well as to evaluate its flexural performance and investigate the strength of brittle materials, such as ceramics. The bending strength is determined in accordance with the national standard GB/T 6569-2006 by means of the following calculation:

$${\sigma }_f=\frac{3FL}{2b{d}^2}$$

where ${\sigma }_f$ is the bending strength, F is the maximum load corresponding to the specimen when it breaks, L is the lower span of the fixture, b is the width of the specimen, and d is the height (thickness) of the specimen parallel to the loading direction.

The stress–strain curves of B₄C/Al composites with different B₄C particle sizes are shown in Figure 4. When the particle size of B₄C is 0.2 μm, 0.5 μm, and 1 μm, the composite curve exhibits a continuous upward trend, whereas the curve for B₄C particles with a size of 10 μm shows that the upward trend gradually tends to moderate. The results indicate that the toughness of the 10 μm B₄C composite surpasses that of the first three samples.

Simultaneously, four composites indicate different degrees of fracture, and the fracture strain increases with the B₄C particle size, which is 0.52%, 0.81%, 1.04%, and 1.34%, respectively. The increase in B₄C particle size leads to an increase in the content of ductile Al phase within the composite matrix, thereby enhancing the fracture strain of the composites.

The peak load and bending strength of B₄C/Al composites with a B₄C particle size of 0.2 µm exhibit the lowest values, which are 481.1 N and 320.0 MPa, respectively. By increasing the B₄C particle size, the peak load and bending strength of B₄C/Al composites increase and reach 839.7 N and 567.4 MPa as the B₄C particle size is 1 µm. However, with the continuous increase in B₄C particle size to 10 µm, the peak load and bending strength of B₄C/Al composites decrease. Wei et al. [32] have highlighted that the interface between ceramics and matrix serves as a crucial medium for stress transfer in the composites. The change in bending strength may be attributed to excessively high interfacial strength, which can lead to unrelieved stress between ceramic particles and the matrix, resulting in stress concentration and reduced strength, while insufficient interfacial strength can cause the interface to peel off during loading, leading to decreased strength and fracture toughness [33].

The flexural modulus represents the ratio of bending stress to bending strain, serving as an indicator of a material’s ability to withstand deformation under elastic conditions. A higher flexural modulus indicates a lower resistance to such deformation. Figure 5 shows the relationship between B₄C particle size and Flexural modulus of B₄C/Al composites. The flexural modulus of B₄C/Al composites with a particle size of 0.2 µm is 61.4 GPa. By increasing the particle size to 0.5 µm, the flexural modulus reaches its maximum value of 63.8 GPa. However, as the particle size continues to increase, the flexural modulus of B₄C/Al composites decreases. When the particle size of B₄C is 10 µm, the flexural modulus of B₄C/Al composites reaches its minimum value of 52.3 GPa.

Based on the results of the XRD and BSE, the brittle phases content of Al₃BC and AlB₂ in B₄C/Al composites decreased, and ductile Al phase increased along with the increase in B₄C particle size. This led to a significant decrease in flexural modulus and a substantial increase in deformation ability under bending load.

Fracture morphologies of B₄C/Al composites with different B₄C particle sizes after bending tests are presented in Figure 6. The morphologies of B₄C/Al composites are similar, showing cleavage fractures with the characteristics of a river pattern [34,35]. In Figure 6b,d, a mass of cleavage steps and some crests of tearing [36] are observed, indicating that the fracture modes of B₄C/Al composites with B₄C particle sizes of 0.2 µm and 0.5 µm are mainly brittle ruptures, accompanied by partial ductile rupture of the Al matrix. Based on the previous analysis, it is found that the matrix of composites containing B₄C particles with sizes of 0.2 µm and 0.5 µm mainly consisted of AlB₂ and Al₃BC phases. The existence of numerous brittle ceramic particles in the interior of B₄C/Al composites results in a higher susceptibility to stress concentration, leading to macroscopic crack propagation dominated by brittle rupture of the ceramic phase, ultimately causing specimen failure under low stress conditions; the aforementioned findings are in line with the outcomes reported in the literature [37].

In Figure 6e,f, as the particle size of B₄C increases to 1 µm, the ductile fracture characteristics of the fracture morphology are more obvious. The presence of distinct tearing crests and an increase in both the depth and quantity of dimples suggest a heightened plastic deformation ability for the composites [38,39]. As the particle size of B₄C increases to 10 µm, the fracture morphology of the composites exhibits distinct ductile fracture characteristics, as illustrated in Figure 6g,h. In addition, the size, quantity, and depth of dimples in fracture morphologies of B₄C/Al composites with a B₄C particle size of 10 µm are significantly increased compared to the previous three specimens. This is attributed to the fact that aluminum metal exhibits significant plastic deformation during the failure of composites, which absorbs energy upon fracture and greatly mitigates stress concentration. In conclusion, the plastic deformation ability of B₄C/Al composites is enhanced with the increase of B₄C particle size.

Fracture toughness is an inherent property of a material, which can be used to represent the ability of a sample to prevent crack propagation and is a crucial index to measure the toughness of a sample. The calculation formula for fracture toughness, as stipulated by the national standard GB/T 23806-2009, is as follows:

$$K_{\mathrm{I}C}=\frac{Pl}{w{h}^{\raisebox{1ex}{$3$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}}f\left(\frac{a}{h}\right)$$

$$f\left(\frac{a}{h}\right)=1.93-3.07\left(\frac{a}{h}\right)+14.53{\left(\frac{a}{h}\right)}^2-25.11{\left(\frac{a}{h}\right)}^3+25.80{\left(\frac{a}{h}\right)}^4$$

where P represents the load, l denotes the lower span of the fixture, w stands for the width of the sample, a indicates the depth of pre-opening in the sample, and h indicates its height. The fracture toughness results of B₄C/Al composites with different B₄C particle sizes are depicted in Figure 7, demonstrating a significant enhancement in fracture toughness of B₄C/Al composites compared to pure B₄C (K_IC about 3.7 MPa·m^1/2 [7,8]). When the particle size of B₄C increases from 0.2 µm to 0.5 µm, the fracture toughness of the composites decreases from 8.73 MPa·m^1/2 to 8.43 MPa·m^1/2. The fracture toughness of the composites shows an inflection point when B₄C particle size is 1 µm, suddenly increases to 12.20 MPa·m^1/2 and reaches a peak value (12.75 MPa·m^1/2) when B₄C particle size is 10 µm. The variations in fracture toughness and bending strength of B₄C/Al composites are mutually corroborated, but generally antithetical to the changes in hardness and bending modulus of the composites.

The main determinant of fracture toughness of B₄C/Al composites is the content of ductile Al phase; an increasing Al phase will yield higher fracture toughness of B₄C/Al composites. This corresponds to the fracture strain and morphology characteristics of the composites, while also affecting the fracture toughness and bending strength through interfacial bonding strength. In summary, the fracture toughness of B₄C/Al composites increases as the particle size of B₄C increases.

3.3. High Temperature Compression Properties

The photograph of B₄C/Al composites with different B₄C particle sizes during quasi-static high temperature compression is shown in Figure 8. The fracture characteristic of B₄C/Al composites with B₄C particle sizes of 0.2 µm and 0.5 µm are similar, and the samples display shear fracture along the 45°–55° direction and local fracture along the compression direction. This phenomenon is attributed to the slip of the internal shear band during the deformation process [14]. On the contrary, when the particle sizes of B₄C are 1 µm and 10 µm, the sample appears as a drum with significant bulging in the middle. This is due to the transverse plastic deformation of the sample during the compression process, indicating that the plastic deformation ability is better than in the previous two samples.

High temperature compression deformation as the foundation for various plastic deformation processes provides a basis for deformation methods such as hot extrusion and rolling of composites. Figure 9 shows the stress–strain curves of B₄C/Al composites with different B₄C particle sizes during high temperature compression at a temperature of 500 ℃ and a strain rate of 0.001 s⁻¹ (quasi-static). The peak compressive flow stress of the composites with B₄C particle size of 0.2 µm is about 370 MPa, the peak compressive flow stress increases to 560 MPa when B₄C particle size is 0.5 µm. On the contrary, the peak compressive flow stress of B₄C/Al composites with particle size of 1 µm and 10 µm decreased significantly to 205 MPa and 150 MPa, respectively. In summary, with the change in B₄C particle size, the maximum peak compressive flow stress of composites decreases by 73.21% from the maximum 560 MPa to 150 MPa.

Based on the previous XRD and BSE results, it is evident that the Al content in composites increases with an increase in B₄C particle size. The softening of Al at a temperature of 500 °C renders the B₄C particles within the composite more susceptible to rolling or sliding, thereby inducing instability in the ceramic particle skeleton. Consequently, Al phase deformation becomes the primary mode of deformation for composites under these circumstances. In conclusion, with the increase in B₄C particle size, the composites exhibit a decrease in flow stress and an increase in fracture strain.

4. Conclusions

In this research, B₄C/Al composites with different B₄C particle sizes (0.2 µm, 0.5 µm, 1 µm, 10 µm) were prepared by pressureless infiltration. The results showed that B₄C/Al composites were composed of Al, B₄C, Al₃BC, and AlB₂ phases. With the increase in B₄C particle size, the interface reaction between Al and B₄C was alleviated due to the decreasing contact area between B₄C and Al. This led to a decrease in the production of brittle Al₃BC and AlB₂ phases and an increase in the ductile Al phase in the B₄C/Al composites. Consequently, the actual densities of all composites fabricated by pressureless infiltration are close to theoretical densities. In addition, with the increase in B₄C particle size, the Vickers hardness decreased, bending strength of B₄C/Al composites increased, and fracture toughness and high temperature deformation ability B₄C/Al composites were improved. The results of this experiment can provide theoretical guidance for B₄C/Al composites interface regulation and subsequent hot processing research.